HIGH-RESOLUTION ELECTRON-MICROSCOPY OBSERVATIONS OF TRANSIENT LATTICES FROM FCC TO BCC AT THE AUSTENITE-MARTENSITE INTERFACE

Atomic arrangements of austenite (fcc) and martensite (bcc) near the i nterface in an Fe-Ni-Mn alloy have been observed by high-resolution el ectron microscopy. It is found that atom rows along the close-packed d irections of two phases are completely coherent at the interface, but there exist deformed lattices which are neither fee nor bcc. These lat tices are regarded as transient lattices changing continuously from fc c to bcc lattice. By observation of these transient lattices we can de duce the lattice deformation in the fee to bcc martensitic transformat ion. Tt is concluded that the lattice deformation proposed by Kurdjumo w and Sachs is perfectly consistent with the observed atomic arrangeme nts near the interface. The Bogers and Burgers mechanism is inadequate to explain the present observations. It is emphasized that the austen ite-martensite interface in the present alloy is not a monoatomic laye r but has a width of about 0.7 nm in which the transient lattices are contained.